1. Field of the Invention
The present invention relates to a textured yarn of polyester filaments obtained by a simultaneous draw-texturing process. More specifically, it relates to a false-twist textured yarn suitable for a woven fabric, having properties defined by specific ranges of crimpability and sonic velocity.
2. Description of the Prior Art
False-twist texturing is suitable for processing synthetic filament yarn because it can produce various types of textured yarns by just adjusting the yarn tension, heater temperature, and other process conditions. In fact, more than 70% of all polyester filament yarns supplied to the market of clothing is in the form of false-twist textured yarn.
The false-twist texturing basically comprises heat-setting a twisted thermoplastic yarn to a plastic condition; cooling the same below a glass transition temperature thereof to fix the spiral form of the twisted yarn; and untwisting the same through a false twister. Of these steps, cooling has been believed the most essential for good textured yarn.
Therefore, all conventional false-twist texturing machines are constructed with a long cooling zone between the heater and false twister. Yarn heated by the heater is forced to pass through the cooling zone, preferably a cooling plate, for at least 0.16 second. The mechanism of cooling is described in detail in "Manual of Filament Processing Technique", vol. 1, p. 90 to 93, published by the Textile Machinery Society of Japan.
A recent trend in flase-twist texturing has been for the use of the so-called POY-DTY system. A partially-oriented yarn (POY), of polyester spun at a rate from 2,500 to 3,500 m/min is processed by a draw-texturing machine (DTY machine) in which the yarn is false-twisted simultaneously with drawing at a higher rate than that of the conventional process applied to a full drawn yarn. The processing rate of the conventional process is usually lower than 150 m/min; while that of the DTY process is more than 500 m/min.
Since DTY machines available nowadays are still constructed in accordance with the above-mentioned conventional principle, they have to have a longer cooling zone corresponding to the higher processing rate so as to ensure an equivalent cooling time as the former conventional machines.
On another matter, one of the important functions required for false-twist textured yarn is a higher width shrinkage of a grey fabric made thereof in a relaxation process. This shrinkage gives the finished fabric a good feel. Width shrinkage has been believed to rely mainly on crimpability of the textured yarn.
The present inventors made various attempts to produce textured yarn fabrics having an improved hand regarding both bulkiness and resiliency by means of the conventional POY-DTY system. However, they failed to obtain the desired fabric. Through their attempts, however, the present inventors found that a cause of their failure was attributed to the yarn cooling mode. According to the conventional understanding, the yarn temperature before introduction to a false twister (pre-twister temperature) had to be below the glass transition temperature (Tg) and, if the temperature were higher than Tg, the spiral form of the yarn would be deformed so that the crimpability of the textured yarn would decrease.